Multilayer ceramic components are generally produced as follows. A plurality of unfired ceramic sheets such as dielectric and magnetic materials and a plurality of internal electrode paste layers are alternately laminated to obtain an unfired laminate. This laminate is cut and then fired at a high temperature to thereby obtain a ceramic element. Subsequently, a terminal electrode paste including a conductive powder and an inorganic binder powder such as a glass frit dispersed in a vehicle is applied by one of various methods such as dipping, brushing, and screen printing to end faces of the element where internal electrodes are exposed. After drying, firing is carried out at a high temperature to thereby form terminal electrodes electrically connected to the internal electrodes. Thereafter, a nickel plating layer, and additionally, a plating layer made of tin or an alloy thereof, which has satisfactory solderability, are formed on the terminal electrode as required.
As internal electrode materials, noble metals such as palladium, silver-palladium, and platinum have been used conventionally. However, as required from resource saving, cost cutting, and additionally, demands to prevent delamination and occurrence of cracks caused by oxidative expansion of palladium and the like, base metals such as nickel and copper are used as internal electrodes. For this reason, also in terminal electrodes, nickel and copper, which are likely to form satisfactory electrical connection with these internal electrode materials, are used. In order to prevent the conductivity from decreasing due to oxidization of the base metals constituting the internal electrodes and terminal electrodes, the firing is carried out in a non-oxidizing atmosphere, that is, in an inert atmosphere such as nitrogen and hydrogen-nitrogen or a reducing atmosphere at around 700 to 900° C., for example.
For such a conductive paste for forming terminal electrodes, it is necessary to use reduction-resistant glass stable even in the case of firing in a non-oxidizing atmosphere, as an inorganic binder. In the case of electroplating a terminal electrode film, an acidic electroplating solution may degenerate or dissolve a glass component to destroy the structure of glass. Thus, adhesion strength between the terminal electrode film and the ceramic element may significantly decrease.
Therefore, glass which has satisfactory acid resistance and is unlikely to be corroded by an acidic plating solution is required, and conventionally, reduction-resistant glasses such as glasses based on barium and on zinc have been mainly contemplated.
For example, in Patent Literature 1, base metal terminal electrodes of a multilayer ceramic capacitor are described, in which electrodes, reduction-resistant glasses such as barium borate glass, barium zinc borate glass, and barium zinc borosilicate glass are employed. Patent Literature 2 mentions that zinc borosilicate glass having a specific composition containing an alkali metal component and an alkali earth metal component is used for forming copper terminal electrodes. Patent Literature 3 mentions that aluminum strontium borosilicate glass is used for forming terminal electrodes.
Terminal electrodes formed by use of these glasses, however, has a problem in that its adhesion strength to the ceramic element is insufficient and, in particular, side portions having a small film thickness are likely to delaminate.
Additionally, a fired film to be obtained is porous, and thus, is susceptible to infiltration of a plating solution when the terminal electrodes are electroplated. As a result, the adhesion strength between the terminal electrodes and the ceramic element is significantly reduced, and cracking or reduction in the insulation resistance of the element is caused. Moreover, there has been a problem in that such infiltration leads to a so-called popcorn phenomenon, in which moisture in the plating solution evaporates and expands due to heating during component mounting to thereby cause a rupture of the terminal electrodes, reducing the reliability of the multilayer ceramic component.
As other glasses for forming terminal electrodes having satisfactory reduction resistance, alkali earth borosilicate glass described in Patent Literature 4 and alkali borosilicate glass described in Patent Literature 5 have been contemplated, but neither of them has sufficiently solved the problems aforementioned.
Patent Literature 6 by the present applicant discloses a glass composition for a thick film paste to be fired in a non-oxidizing atmosphere, which composition is free of lead, cadmium, or bismuth and comprises 35 to 60% by mass of BaO, 5 to 35% by mass of B2O3, 0 to 12% by mass of ZnO, 2 to 22% by mass of MnO2, 0 to 18% by mass of Al2O3, 0 to 11% by mass of SiO2, 0 to 8% by mass of one or more selected from Li2O, Na2O, and K2O, 0 to 10% by mass of one or more selected from Cu2O, SnO2, Fe2O3, and Co3O4, 1 to 25% by mass of TiO2, and 0 to 5% by mass of ZrO2. Use of this glass composition can form dense terminal electrodes having sufficient adhesion strength to a substrate.